Microphone



March 6, 1951 A. H. KETTLER 2,544,536

MICROPHONE Filed May 28, 1947 I INVENTOR Amezafl irmse BYGQ ATTORNEY Patented Mar. 6, 19 51 MICROPHONE Alfred H. Kettler, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 28, 194v, Serial No.- 751,007

12 Claims. (Cl.

This invention relates to microphones, and more particularly toa pressure gradient responsive microphone whichhas highly desirable discriminating characteristics against unwanted sounds. 5

'The" undesirable pick up of noise by' microphones has .long. been '-.a problem, particularly in aircraft, engine-rooms, engine test rooms, power, plants, etc. It hasxalso been the experience of users of microphones that the undesirable pick 19 up of the speech ofnearby persons in announce booths, paging booths, offices, etc., is annoying andifrequently embarrassing. Similarly; the undesirable'pick up of amplified'speechin public address" or amplifier systems which .leads to.

feed-back" (whistles, howls, etc.) under conditions of high amplification has also been a, source of considerable annoyance. In prior at-. tempts to solve these and other similar problems in connection with the pick up of sound by microphonesvarious types of microphones have been proposed, among them being the pressure operated microphones, throat microphones, pressure gradient or -velocity microphones of the first order, and second order gradient microphones. :"However, each of these is subject to one or another disadvantage: which renders it not entirely suitable." 5 :Of the several 'types of prior art microphones mentioned in the preceding paragraph, perhaps 3 the most satisfactory'is a second order gradient microphone of thetype disclosed in the Olson Patent, 2,301,744. 'In a microphone of this type,; the difference in output of two first order pres-..

sure gradient microphones placed very close to-I 5 gether is employed so as to give, effectively, the pressure gradient of the pressure gradient." This: arrangement has the advantages. that it provides" considerably. less responseto distant noises than the first order gradient type of microphone, and 40 also that it provides considerably less response to random direction noise than the latter type of microphone. Because of its. improved discrimination, this second order gradient microphone'need not be used directly against the-lips or against the throat of the user; as with the other types; -It is, however, amorencomplicated structure than any of the other microphones mentioned. above, requiring twounits which should be asnearly' identical: insensitivity and frequency response as possiblajand also requiring small size so thatthe spacing between the individual first order gradient microphone unitsmay be made very small. Another disadvantage of thistype of second order gradient microphone up 179- -1-15-5), is that the air stream may also cause disturbing. noises.

The primary object of my present invention is to provide an improved pressure gradient responsive microphone which can be operated to provide a second order gradient effect and which L will be free from the above mentioned and other disadvantages characteristic of art. microphones. Moreparticularly, it is an object of my present invention to provide an improved pressure. gradient responsive microphone which will have. superior discriminating characteristics againstunwanted sounds, particularly those arising at a distance of several feet or more.

Another object of my present invention is to provide an improved pressure gradient responsive microphones as aforesaid, which, while offer-'- ing performance at least equal to that of the prior 0 above noted Olson microphone, is nevertheless of greatly simplified construction and can be manuf actu'red at a much lower cost.

Still another object of my present invention is .to provide an improved pressure gradient responsive microphone as above set forth which. has a substantially unidirectional characteristic, particularly at the higher frequencies. 1 It is also an object of my present invention to 1 provide an improved pressure gradient responsive microphone as above set forth which is very simple in construction, economical in cost, and highly efficient in use.

In accordance with .my present invention, 1- make use of but a single first order pressure gradientor velocity responsive microphone, and place in' close proximity to one side of the vibratile member thereof a reflector, preferably in the form of a disk made of rigid, non-resonant. material- This reflector disk has an area which is considerably larger than that of the vibratile member of the microphone. The microphone is small (the size limitation being, for example, the high frequency performance which is desired),- and. it is so constructed that its moving element may be placed close to the reflector. The arrangement is such that a sound wave from a source external to the microphone can pass di-" rectly to both surfaces of the vibratory elementthereof. The sound wave is also reflected back to the vibratory element by the reflector which is in close-proximity to it. Thus, the vibratory element can be actuated simultaneously by the pressure gradients of both the direct sound andthe reflected sound, so that, the pressure gradient ofthese pressure gradients provides the force which moves the movable element. It is apparent, therefore, that the movable element of the microphone is moved according to a second order pressure gradient effect.

The distance between the movable element of the microphone and the reflector should be small compared to the focal length of the reflector. In one microphone constructed in accordance with the present invention, this distance was of the order of A3, although it could be made either smaller or larger depending upon the frequency below which best discrimination is desired. In general, the smaller this distance is, the higher this frequency. The reflector should preferably be of non-resonant material, approximately 4 thick and of the order of 6" in diameter where, for example, the movable element. or diaphragm of the microphone is of the order of /4. in diameter. The reflector may be in the form of a flat, transparent disk arranged coaxially with the diaphragm. A disk made. of transparentmaterial, such, for example, as certain polymerized methyl methacrylate resins available in the market in transparent form, has the advantage that it ofiers no obstruction to vision when the user is looking downward, the disk reflector being usually out of the line ofsight when the speaker is looking straight ahead. If desired, they re.- flector may be made concave or convex, the degree of curvature determining speech diffracting and focusing effects. reflector may be mounted off center with respect to the diaphragm axis so: as to avoid symmetry and so to provide increased vision clearance.

The novel features of m present invention, both as to. its organization and method of operation, as well as additional: objects and ad.-. vantages thereof, will better be understood from the following description, when read in connection with the accompanying drawing in which Figures 1 and 2 are diagrammatic views, in side elevation, of av microphone in accordance with my present invention, these figures being referred to hereinafter for the purpose of explaining the principle involved in its operation,

Figure 3 is a. front elevation of one form of microphone constructed in accordance with my present invention,

Figure 4 is a side elevation thereof,

Figure 5 is a. sectional view taken on the line VTV of Fig. 3 with they wind screen. omitted,

Figure 6 is an enlarged, fragmentary sectional viewof a, portion of the structure shown in Fig. 5, and

Figures '7 and Bare diagrammatic views, in side. elevation, showing slightly diflerent forms: of; microphones in accordance. with my. present in-. vention.

Referring more particularly, first, to Figs. 3 to 6, inclusive, there. is. shown a pressure gradient responsive microphone having a field structure consisting of a magnet I a center pole piece 3- thereon, an annular pole piece 5 arranged con centrically with the pole piece 3 and spaced therefrom to provide an air gap I, and a plurality ofstuds 9 of magnetic material which connect the annular pole piece 5 with the base of themagnet I: Suitably supported on the annularpole piece 5 between a pair of clamping rings I is- 9. vibratory diaphragm I I which is provided with a voice coil or conductive element I'3- disposed in the air gap I in well-known manner. The microphone is provided with a cap I having aplurality of circumferentially spaced slots oropenings Il therein through which sound waves from If desired, also, the a face of the diaphragm II. A screen I9 of silk or the like may be placed over the openings I1 and held thereon by a suitable clamping ring M which is provided with circumferentially spaced openings 23 corresponding in number and size to the openings IT. A second screen 25, also of silk or the like, may be held in place behind the air gap I by a pair of clamping rings 21. At the same time that sound waves from an external source reach the front surface of the diaphragm I I. through the openings H, the sound waves may reach the rear surface of the diaphragm H through a plurality of circumferentially spaced, aligned openings 29 in the clamping rings 21. It will be apparent, therefore, that the instantaneous differences of sound pressure on the opposite sides of the diaphragm II will cause the diaphragm to vibrate and thus produce signal currents in the voice coil I3. This is the manner of operation of the well-known, first order pressure. gradient or velocity type microphones. A wind screen 30 of silk or any other suitable material may be placed over the cap I5, if desired.

In accordance with my present invention, I place in proximity to the rear surface. of the diaphragm II a reflector 31 for reflecting back tothe diaphragm I I sound waves which are emitted by and reach it from the above mentioned soundsource. The reflector BI is preferably in the form of a disk made of a rigid, non-resonant material. In order to place; the disk 3I in close proximity to. the rearsurface of the diaphragm, II-, the disk may be. mounted onthe field structure of the microphone. with the center pole piece 3 and the. studs 9 extending therethrough. With this. arrangement, it will be noted that the magnet I and the major portion of each of the studs. 9 are behind the. rear surfaceof the. disk 31 while the pole pieces. 3. and 5. and. the. diaphragm H are disposed in front of the front. surface of the disk: 3I. reflector 3 I may be. designed into. the. magnet as sembly and the. whole made a. removable, central section of the reflector 3|- The theory or principle. of operation of my improved microphone. may be readily undersimod by reference to Figs. 1 and 2 wherein the dia.-- phragm II. and the. reflector 3.1 are shown diagrammatically. Let it be. assumed that sound is. emitted by asource 81- front of the; diaphragm H and that; the sound wave will have. a.v wave. front represented by the circles. 33, 35, expand; ing, in the directions of the. arrows; 3.1. When this sound wave strikes. the reflector'SI, it iszreflected thereby back toward the point P along a wavefront. 39' which has. an image source. S2. If the diaphragm II; is located' at the point. P spaced a. distance. d: from, the reflector 3;I. (Fig. 2') there. is. a. path. difierence equal to dz.-d1 between the distance of the: real source $1 from the point P' and the distance of the. image source S2 from the point. P. This path diiference is equal to twice the spacing of the point P from the reflector 3I The location of the image source S: with reference to the point P (that is, the distance d2) may bevaried or controlled by the curvature of the reflector 3 I soasto give greater. or less than twice the spacing from: the point? to the reflector-asthepath difference. Thus; the reflector 3I may: bemade. concave relative to the diaphragm II', as illustrated in Fig; 7-, or its reflecting' surface may be made convex relative to the diaphragm I I.

If the microphone is of the first order pres- If desired, the central portion of. the

'i sure gradient'type, as in the present case, the output of the microphone will .be reduced .due to the presence of the reflector since the particle velocity of a sound wave is reflected from a rigid surface with a reversal of phase. The output of the microphone is thus due to the difierencelin the pressure gradients of the sound wave at two points separated by a distance 2d in the case of a flat reflector. More precisely, the force actuating the diaphragm may be represented. by the following equation:

i2 in F dx (1x where xthe distance as measured from the sound source S to the diaphragm. p

dx =the derivative of the sound pressure oftheincident wave with respect to distance (pressure It is understood, however, that the expression g g dxz is negative as compared to the expression because of the phase reversal due to reflection of the sound wave from a rigid surface. Consequently, th actuating force for the diaphragm is the difference between two pressure gradients separated a distance 2d. As a result, it will be recognized that this provides the same result as that obtained by the use of two oppositely phased pressure gradient units spaced apart a distance 2d. As taught by the above-mentioned Olson Patent 2,301,744, this defines a second order gradient microphone. In other words, the pressure gradient of the pressure gradients of the sound wave is obtained for actuating the diaphragm.

In one microphone constructed in accordance with the present invention, the reflector 3| was a plane disk of transparent resin such as specified above approximately 6" in diameter and the vibratory portion of the diaphragm was approximately 4" in diameter. The disk 3| was about /a" thick and spaced from the diaphragm ll about 3%". This microphone had a fundamental resonance at approximately 1100 C; P. S. and a secondary resonance at from 4000 to 5000 C. P. 8., both suitably damped. The above frequencies were chosen for the type of response characteristic that was desired. In this microphone, the following improvement was measured:

(1) Front side--on the microphone axis:

95 cycles/second, 13 decibels 200 cycles/second, 15 decibels 500 cycles/second, 16%; decibels 1000 cycles/second, 14 decibels 2000 cycles/second, 6 decibels 3600 cycles/second, 0 decibels =the derivative of the sound pressure of the re- (2) Back side-on the microphone axis:

Below 1000 cycles/secondsame as front side 2000. cycles/second, 13 decibels below front response 3600 cycles/second, 18 decibels below front response (3) At various angles:

Below 1000 cycles/second, the directional pattern approximated to the cosine squared characteristic. Above 1000 cycles/second, there was a decided unidirectional effect, the response at the rear being as much as 25 db be-- low response at the front at some ire-- quencies.

B. Total discrimination:

To obtain the total discrimination against distant sound as compared to a pressure microphone used at a speaking distance of inch from the microphone, the advantage given under (1) and (2) above must be added to that obtained by using the first order gradient unit, The latter is quite appreciable, being approximately 20 db at 500 cycles/second and 12 db at 1000 cycles/second. Thus, the total discrimination is approximately 36 db at 500 cycles/second (more at lower frequencies) and 26 db at 1000 cycles/second, de-' creasing with increasing frequency.

From the foregoing description, it will be apparent to those skilled in the art that I have provided an improved, second order pressure gradient microphone of very simple construction which is highly effective in discriminating against unwanted sounds. Although I have shown and described but a single embodiment of my invention, it will undoubtedly be apparent to those vention. For example, as pointed out heretofore,

and as illustrated in Fig. 8, the reflector 3| may be disposed off center with respect to the axis of the diaphragm II so that the axis of the disk 3|, While parallel with that of the diaphragm -I l, is spaced laterally therefrom. This will provide better vision clearance at the top. If desired, the

reflector 3| may be so disposed that its axis will be an'gularly related to that of the diaphragm I I.

' Furthermore, it will undoubtedly be apparent P that a reflector in accordance with the present invention may be employed in connection with a first order pressure gradient responsive microphone which utilizes a vibratory conductive ribbon, as disclosed, for example, in the Olson Patent 1,885,001. Also, if desired, the reflector 3| may be made, at least in part, of acoustically absorbent or semi-acoustically-conducting ma-' terial to obtain certain phase shifting and/oi directional effects. Various other changes will, no doubt, readily suggest themselves to those skilled in the art. I therefore desire thatthe above description shall be taken as illustrative and not as limiting.

I claim as my invention:

1. A microphone comprising means for producing a magnetic field, said means including an air gap,'a vibratile member mounted for vibration in response to instantaneous differences of sound pressure on opposite sides thereof, said vibratile member including a conductive element disposed in said'air gap,'the vibrations of said conductive element in said air gap serving to convert vibrations of said vibratile member into electrical variations, and an acoustical reflector disposed in close proximity to one side of said member, the spacing between said member and said reflector being small compared to the focal length of said reflector.

.2. A microphone comprising means for .producing a magnetic field, said means including an air gap, a vibratile member mounted for vibration in response to instantaneous differences of sound pressure on opposite sides thereof, said vibratile member including a conductive element disposed in said air gap, the vibrations of said conductive element in saidair gap serving to convert vibrations of said vibratile member into elec trical variations, and an acoustical reflector dis-- posed in close proximity to one side of said member, the spacing between said member and said reflector being small compared to the focal length of said reflector, said reflector having an area which is large compared to that of said vibratile member.

3. A microphone comprising means for producing a magnetic field, said means including an air gap, a vibratile member mounted for vibration in response to instantaneous differences of sound pressure on opposite sides thereof, said vibratile member including a conductive element disposed in said air gap, the vibrations of said conductive element in said air gap serving to convert vibrations of said vibratile member into electrical variations, and an acoustical reflector disposed in close proximity to one side of said member, the spacing between said member and said. reflector being small compared to the focal length of said reflector, said reflector having an area which is large compared to that of said vibratile member, and the axes of said member and said reflector being coincident.

4. A microphone comprising means for producing a magnetic field, said means including an air gap, a vibratile member mounted for vibration in responseto instantaneous dilferences of soundpressure on opposite sides thereof, said vibratile member including a conductive element disposed in said air gap, the vibrations of said conductive element in said air gap serving to convert vibrations of said vibratile member into electrical variations, and an acoustical reflector disposed in close proximity to one side of said: member, said reflector having an area which is large compared to that of said vibratile member, and the axes of said member and said reflector being laterally offset from each other.

5. A microphone comprising means for producing amagnetic field, said means including an air gap, a vibratile member mounted for vibration in response to instantaneous diiferencesof; sound pressure on opposite sides thereof, said vibratile member including a conductive elementdisposed in said air gap, the vibrations of'saidconductive element in said air gap serving to convert vibrations of said vibratile member into electrical variations, and an acoustical reflector disposed in close. proximity to one side of said member, said reflector having an area which is large compared to that of said vibratile member, and the axes of said member and said reflector being laterally offset from but parallel to each other.

6. A microphone according to claim 1 wherein said reflector is rigid.

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7. A'microphone according to claim 1 wherein said reflector is of. non-resonant material.

8. A microphone comprising a, magnetic field structure having an air gap, a vibratile member including a conductor mounted for vibration in said air gap, said member being responsive to the pressure gradient component of a sound wave,

and an acoustical reflector supported by said field structure in close proximity to one side of said member for reflecting back to said member sound waves reaching said reflector from a source ex-'.

ternal thereto, the spacing between said member and said reflector being small compared to the focal length of said reflector.

9. A microphone comprising a magnetic field structure having an air gap, a vibratile member including a conductor mounted for vibration in. saidair gap, said member being responsive to a the pressure gradient component of a sound wave, and'an acoustical reflector supported by said field structure in close proximity to one side of said member for reflecting back to said member sound waves reaching said reflector from a source external thereto, said field structure ex-" tending through said reflector.

10. A microphone comprlsing a magnetic field structure having a magnet and a pair of pole pieces associated with said magnet and spaced from each other to provide an air gap, a vi-' bratile member including a conductor mounted for vibration in said air gap, said member being responsive to the pressure gradient component of a sound wave, and an acoustical reflector supported by said field structure in close proximity to one side of said member for reflecting back to said member sound waves reaching said reflector from a source external thereto, said pole pieces being disposed on one side of said reflector and said magnet on the other side thereincluding a conductor mounted for vibration in said air gap, said member being responsiveto the pressure gradient component of a sound wave, and an acoustical reflector comprising adisc of non-resonant material supported by said field structure in close proximity to one side of said member for reflecting back to said member sound waves reaching said reflector from REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Date Number Name 1,430,256 Pridham et al. Sept. 26, 1922 1,885,001 Olson et al. Oct. 25, 1932 1,899,994 Spotts' Mar. 7, 1933 1,975,283 Mueller Oct. 2, 1934 2,049,586 5 Hanson et a1 Aug. 4, 1936 2,085,721g Wornke June 29, 1937 2,352,305 Anderson June 27, 1944 11. A microphone comprising a magnetic field structure having an air gap, a vibratile member 

